At the beginning of this poem, the sound of the lines rolls along, steady and rhythmic. We think there's something really comforting about the stable, calm sound of the opening stanza. It's like being inside a cozy cabin on the beach, and listening to waves crash outside. It's a little bit grand and exciting, but totally safe, too. Take these lines for example:
Banners yellow, glorious, golden,
On its roof did float and flow, (9-10)
Not only is the meter really calm and regular, but also there's an open, relaxed sound to the words. Notice the repetition of the vowel sound (in fancy poetry terms we call that assonance). In word after word, you get that open O sound: "yellow," "glorious," "golden." We just "float and flow" along in these opening sections, letting the words carry us along.
Then, of course, everything changes. Suddenly things go bad, and the palace is taken over. The words get a little choppier (like the word "discordant," which both means and sounds "out of tune). Things seem to speed up, and now the "ghastly rapid river" (a phrase that features some short A assonance in "ghastly" and "rapid") hurries us to our doom. The rhythm isn't so different, but the sound and the meaning of the words are completely new. Consider the short, clipped vowel sounds in "the red-litten windows," for example (42). It's as if we were hearing the poem in a new way. Instead of being safe inside, away from the waves, now we're out on the stormy sea, in a boat that's about to sink (and it's full of snakes, and it's on fire… oh sorry, we got carried away there—must be all this Poe).